A communication system is utilized to communicate data between a sending station and a receiving station. Data that is to be communicated is sourced at the sending station and then communicated upon a communication channel to the receiving station at which the data is terminated.
The data is sometimes modulated, or otherwise altered, to place the data in a form to permit its communication upon the communication channel. And, once received at the receiving station, the data is demodulated, or otherwise operated upon, if necessary, to recover the informational content of the data.
As technological advancements have permitted, different types of communication systems have been developed. Many different types of communication systems are regularly utilized to effectuate the communication of data to perform many varied types of communication services. New types of communication systems, as well as improvements to existing communication systems, shall likely continue to be developed and implemented.
An exemplary type of communication system is a radio communication system. In a radio communication system, the communication channel upon which data is communicated between the sending and receiving stations is formed upon a radio link. A radio link is a designated portion of the electromagnetic spectrum, sometimes defined in terms of the frequency at which the radio link is defined. A fixed, wireline connection is obviated for the portion of the communication path extending between a sending and a receiving station that utilizes, instead, radio channels. Radio communication systems, as a result, are sometimes less expensive to install, relative to wireline counterparts, as a result of reduced infrastructure costs. And, a radio communication system is amenable for implementation as a mobile communication system in which communication mobility is permitted.
The allocation amount of the electromagnetic spectrum available upon which to define radio links at which to form communication channels is generally limited. Radio communication systems are, therefore, regularly referred to as being bandwidth-constrained systems. Sometimes, only by more efficiently utilizing the allocated spectrum can the communication capacity of the communication system be increased.
Use of digital communication techniques, for instance, provide for the possibility to increase the communication capacity of a communication system. Many digital communication techniques format digitized data into packets or frames and communicate the packets or frames upon shared communication channels, permitting packet-switched communications to be utilized.
Communication services that are, or are planned to be, effectuated are increasingly data-intensive. Large amounts of data must be communicated between sending and receiving stations in timely, and accurate, manner, properly to complete the communication service.
Realizable communication systems are nonideal. That is, the data that is communicated during operation of the communication system is distorted in some manner during its communication between its source at a sending station and its destination at a receiving station. For instance, in many, if not all, nonideal mobile radio communication systems, frequency offsets are introduced upon data symbols forming the data as the data is communicated between sending and receiving stations. The frequency offsets are caused by Doppler shifting when at least one of the sending and receiving stations is moving at a velocity during communication of the data. As the relative velocity increases, the frequency offsets caused by the Doppler shifting generally increase.
The frequency offset causes rotation of the received data symbols, when detected at a receiving station. That is to say, a data symbol is defined, in part, by a frequency component. And, the frequency offset caused by the Doppler shifting rotate, that is, alter, the frequency component of the data symbol. The offset must be estimated, and thereafter compensated for, to permit the actual values of the symbols properly to be recovered.
Additional frequency offset is introduced at the receiving station that receives the data. Frequency offset is introduced here, for instance, as a result of electrical circuitry imperfections. Frequency offset is introduced at the receiving stations of both mobile communication systems as well as other fixed-site receiving stations. Again, the frequency offset must be estimated, and thereafter compensated for, to permit the actual values of the symbols to be recovered properly.
In one manner by which to estimate the offset, pilot symbol assistance is utilized. Pilot symbols are contained in a data burst and are used to make a maximum likelihood (ML) estimation. In order for the estimation to be accurate, generally, a large number of pilot symbols are required to be contained in the burst. This need is especially evident if the carrier frequency is random as in a frequency hop environment or when the velocity at which a communication station is moving varies. In such a situation, the estimate can not be averaged over multiple bursts. The need to utilize a large number of pilot symbols reduces the number of symbols that can form data symbols in the bursts. And, thereby, the communication capacity of the communication system is limited by the need to utilize the large number of pilot symbols.
Sometimes, the maximum likelihood estimation that utilizes pilot symbols is supplemented by decision-directed symbols, based upon data symbol detection. However, this method works poorly, or fails, if the magnitude of the frequency offset is too large to permit the detection to yield accurate estimation of the data symbols. This type of estimation technique, therefore, is of limited utility when communication conditions induce large amounts of frequency offset.
In another manner by which to estimate the offset, estimation does not detect data symbols but instead utilizes the data symbols blindly. Methods that utilize this type of estimation usually are computationally complex. And, the computational capability of some communication stations is limited. This manner of estimation is, therefore, unavailable, or impractical, in systems in which computational capacity is limited.
As these existing manners by which to estimate the frequency offset suffer from various disadvantages, an improved manner by which to estimate frequency estimates would permit improved estimations to be performed, thereby improving the quality of communications in the communication system.
It is in light of this background information related to manners by which to estimate frequency offset in a communication system that the significant improvements of the present invention have evolved.